In a fiber laser, the resonator is composed of a specific fiber that contains a single mode fiber in the inside core this being matched in terms of dimensions and material to the wave length range of the laser to be achieved and the diameter thereof lying in the region of a few μm. This “laser fiber” is surrounded by a “pump fiber” having a diameter of a few hundred μm into which the pump light is coupled. The “laser fiber” is thus embedded into the core of the pump fiber. The pump fiber is surrounded by a sheath of material having a different refractive index that guarantees the guidance of the pump light in the pump fiber, as known from light waveguide technology. The core of the pump fiber can have a round cross-section but can also have a cross-section deviating therefrom, for example rectangular or quadratic, in order to enable an especially good matching to the pump source (laser diode).
The pump mechanism occurs in that the pump light excites the laser fiber. As a result thereof, the pump energy is consumed more and more over the length of the fiber, namely, beginning at the pump source, the energy content of the pump fiber drops roughly exponentially up to the end, i.e. to the laser exit. Optical efficiencies of above 50% are achieved with fiber lasers. For that purpose, fiber lengths of approximately 50 meters are required. Up to 90% of the pump light has been consumed by the end of the fiber. Due to the exponential consumption of the pump power, it is not meaningful for economical reasons to make the pump fiber even longer, i.e. approximately 10% of the pump light emerges from the pump fiber and is superimposed on the laser light from the inner core of the fiber; the laser light thereby emerges from the fiber as a thin, diffraction-limited bundle, whereas the pump light has a very large aperture angle.
The wave length of the pump source of a known fiber laser lies at 900 nm; the wave length of the corresponding laser lies at 1100 nm. The pump power of this laser amounts to 20 W; the laser power amounts to approximately 10 W. Approximately 2 W pump power are superimposed on the laser light.
Given applications that attach importance to a precise laser power on the order of magnitude of 1%, as is generally required, for example, in reprographics, the presence of the pump light leads to considerable problems, since it does not follow the beam path of the laser light because of the different aperture. Substantial measuring errors in the sensors thus occur due to stray light that the pump light causes. Likewise, inadmissible heating by the pump light occurs in sensitive arrangements.
Although the pump light could be separated from the laser light by a steep edge filter, the filters are easily destroyed given high power densities. This leads to a spatially large structure and expensive filters. It would likewise be conceivable to intercept the pump light with suitable diaphragms. The problem with this is that either the diaphragms must be made so large that they also allow pump light to pass or there is the risk that the diaphragms burn given slight mis-adjustment.